Refactor parse_datetime to only expose parse_datetime function

Create new file parse_relative_time.rs with the relative time helper function.
Renames from_str to parse_datetime and parse_relative time.
Adds function parse_datetime_at_date.

fuzz/fuzz_targets/parse_datetime.rs

@@ -4,5 +4,5 @@ use libfuzzer_sys::fuzz_target;
 
 fuzz_target!(|data: &[u8]| {
     let s = std::str::from_utf8(data).unwrap_or("");
-    let _ = parse_datetime::from_str(s);
+    let _ = parse_datetime::parse_datetime(s);
 });

fuzz/fuzz_targets/parse_datetime_from_str.rs

@@ -1,8 +0,0 @@
-#![no_main]
-
-use libfuzzer_sys::fuzz_target;
-
-fuzz_target!(|data: &[u8]| {
-    let s = std::str::from_utf8(data).unwrap_or("");
-    let _ = parse_datetime::parse_datetime::from_str(s);
-});